During prophase 1, chromosomes supercoil and condense. They shorten and thicken and become visible under a light microscope as distinct structures.During the S phase of interphase the DNA has replicated so the appearance of the chromosomes is as two sister chromatids joined by a centromere.The nucleolus disappears, the nuclear membrane disintegrates and the centrioles move to opposite poles.Homologous chromosomes pair up and the pairs are called bivalents.The bivalents exchange genetic material by crossing over (entanglement) at points called chiasmata during a process called synapsis. The chromatids cross over, become entangled and break. When they rejoin parts of the chromatids have been exchanged.